1. Field of the Invention
The present invention relates to a contact-type charging unit and a contact-type image transfer unit for use in an image forming process such as an electrophotographic process using an electrophotographic photoconductor, which can also be employed, for instance, in an electrophotographic copying machine, facsimile apparatus, laser printer, and direct digital printing master making apparatus.
2. Discussion of Background
The electrophotographic process using an electrophotographic photoconductor, which is utilized in electrophotographic copying machine, facsimile apparatus, laser printer and direct digital printing master making apparatus, includes at least the steps of conducting first charging for uniformly charging the surface of the photoconductor, exposing the charged surface of the photoconductor to light images to form latent electrostatic images thereon, developing the latent electrostatic images with toner to make visible toner images, transferring the toner images to a transfer sheet, fixing the toner images to the transfer sheet, and cleaning the surface of the photoconductor.
Charging and image transfer methods in the electrophotographic process can be classified into two methods, that is, a non-contact method and a contact method.
In the non-contact method, an electroconductive member in the form of a wire or a plate, such as a corona charger, is fixed out of contact with the photoconductor in parallel therewith, and a high voltage is applied to the electroconductive member to charge the photoconductor, whereby charging and image transfer are carried out. Of the various conventional methods, this non-contact charging method is in most general use because uniform charging of the surface of the photoconductor can be carried out relatively easily.
In contrast to the above, the charging or image transfer in the contact method is carried out by bringing an appropriate electroconductive, elastic brush, roller-shaped brush, blade or belt into contact with the surface of the photoconductor with the application of a voltage thereto as disclosed in Japanese Laid-Open Patent Applications 63-149668 and 7-281503.
The contact charging or image transfer method is recently becoming very prevalent, because the voltage required to be applied to the photoconductor for conducting the charging or image transfer is lower than that required for the non-contact charging and also because the generation of ozone, which is considered to be chemically harmful to the photoconductor and the human body, is minimal in the course of the charging or image transfer.
In line with the trend toward the personal use of the copying machine, facsimile apparatus and laser printer, there is an increasing demand for the reduction in size of such electrophotographic apparatus and the improvement of durability thereof so as to be free of maintenance.
However, various units are required to be positioned around the photoconductor. Therefore, when the size of a photoconductor is reduced by decreasing the diameter or outer periphery of the photoconductor in order to obtain a small-sized electrophotographic apparatus, there is caused a problem that the arrangement of the units around the photoconductor becomes extremely difficult.
For instance, a technology of disposing around the photoconductor means for supplying a lubricity-imparting agent to the surface of the photoconductor is disclosed in Japanese Laid-Open Patent Applications 6-342236, 8-202226 and 9-81001. It is evident that the disposing of such a peripheral unit around the photoconductor makes it difficult to reduce the size of the apparatus employing the electrophotographic process.
As a photoconductor for use in electrophotography, organic photoconductors are now widely used because of various advantages over other photoconductors, for example, because of low manufacturing cost, high degree of freedom in the designing of the photoconductor, and no pollution problems.
Examples of conventionally known organic photoconductors are photoconductive resin as represented by polyvinylcarbazole (PVK), charge-transport complex type photoconductors as represented by PVK-TNF(2,4,7-trinitrofluorenone), pigment-dispersion type photoconductors as represented by a phthalocyanine-binder photoconductor, and function-separated type photoconductors composed of a charge generation material and a charge transport material in combination. Of these conventional organic photoconductors, special attention is particularly paid to the function-separated type photoconductors.
A mechanism for the formation of latent electrostatic images on the function-separated type photoconductors is such that when the photoconductor is charged and then exposed to light images, the light passes through a transparent charge transport layer and is absorbed by the charge generation material in a charge generation layer, and the charge generation material which absorbs the light generates charge carriers, and the thus generated charge carriers are injected into the charge transport layer and are transported through the charge transport layer in accordance with an electric field which is generated by charging, whereby the charges on the surface of the photoconductor are neutralized. Thus, latent electrostatic images are formed on the surface of the photoconductor.
In such a function-separated type photoconductor, it is conventionally known and considered to be effective to use in combination (a) a charge transport material which exhibits main light absorption in an ultraviolet region and (b) a charge generation material which exhibits main light absorption in a visible region.
Such organic function-separated type photoconductors are extremely useful for reducing the occurrence of defects such as image blurring, and attaining a long life by avoiding the deterioration thereof as caused by the exposure to corona products such as ozone and NOx, when subjected to contact charging with high charging effect and minimal generation of the corona products. In light of these merits, many proposals concerning such organic photoconductors for use in the contact charging have been made, for example, in Japanese Laid-open Patent Application 56-104351, 57-178267, 58-40566, 58-139156 and 58-150975.
However, when such organic photoconductors are subjected to contact charging, several problems including, not only a problem of uneven charging, but also other problems as caused by the physical contact between a charger and the photoconductor, have been pointed out.
Most of the charge transport materials that have been developed are low-molecular weight compounds. Such low-molecular weight compounds, however, do not exhibit film-forming properties when used alone, so that the low-molecular weight compounds are usually used in the form of a dispersion or a mixture with an inert polymer to prepare a charge transport material. The thus prepared charge transport layer comprising the low-molecular weight charge transport material and the inert polymeric material is generally soft and lacking in rigidity, and therefore has a shortcoming that the charge transport layer tends to be peeled off in the course of repeated use in an electrophotographic process. For the achievement of the improvement of the durability of electrophotographic engine, there is a keen demand for the solving the above-mentioned problems.